Pin Docker image maven:3.8.5-eclipse-temurin-17

--platform=$BUILDPLATFORM
->
maven:3.8.5-eclipse-temurin-17@sha256:a554849fd74b733acb76379bd7df0ea26635a48ab9b54ede1e94bbeaaf448cf6

 [atomist:generated]
 [atomist-skill:atomist/docker-base-image-policy]

Signed-off-by: Atomist Bot <bot@atomist.com>

sparkjava-mysql/backend/Dockerfile

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1.4
 
-FROM --platform=$BUILDPLATFORM maven:3.8.5-eclipse-temurin-17 AS build
+FROM --platform=$BUILDPLATFORM maven:3.8.5-eclipse-temurin-17@sha256:a554849fd74b733acb76379bd7df0ea26635a48ab9b54ede1e94bbeaaf448cf6 AS build
 WORKDIR /workdir/server
 COPY pom.xml /workdir/server/pom.xml
 RUN mvn dependency:go-offline